1. Field of the Invention
This invention relates to an apparatus for expanding a compressed motion picture signal reproduced from a recording media such as a compact disc, cassette tape, and optical disc etc.
2. Description of the Prior Art
Conventionally, since recording a motion picture signal requires that an extremely large amount of information be recorded, a recording medium capable of an extremely high rate of recording and reproduction is required. For example, to record and reproduce an NTSC-standard video signal requires the use of a conventional video disc having a very large information recording capacity. If it is desired to record the motion picture signal on a smaller recording medium having a smaller recording capacity, and to provide a recording time comparable with that of a conventional video disc, the motion picture signal must be compressed using a form of high-efficiency compression prior to recording. Additionally, means for efficiently expanding the compressed motion picture signal must also be provided.
In order to comply with these demands, various high-efficiency systems for compressing motion picture signals have been proposed. One of these systems is known as the MPEG (Moving Picture Experts Group) system.
In the MPEG system, differences between pictures of the motion picture signal are determined to reduce redundancy in the time domain. These differences are then subject to a discrete cosine transform (DCT) to reduce the redundancy in the spatial domain. However, in the MPEG system, the decoder is disposed at an upstream position in the decoding stage. Therefore, if the compressed motion picture signal includes an error which cannot be corrected by the error correcting ability of the decoder, the resulting motion picture output signal lacks a portion corresponding to the erroneous part of the compressed signal. If no error concealment processing is applied to the motion picture output signal, the missing signal portion impairs the quality of the picture displayed in response to the motion picture output signal.
Accordingly, an uncorrectable error in the compressed motion picture signal is conventionally concealed by fitting part of a past picture in place of the missing portion of the current picture. This method is effective when there is no motion in the missing portion in the current picture relative to the past picture. However, the missing portion and portions of the picture surrounding the missing portion will not connect smoothly if there is motion between the current picture and the past picture. Therefore, the picture error resulting from the error in the compressed motion picture signal will still be noticeable, and an acceptable result will not always be obtained.